This invention relates to video signal processing, and more particularly to the combined functions of converting a video signal from analog-to-digital and clamping the video signal to a reference level.
Clamping is performed in conjunction with the analog-to-digital conversion of the video signal in order to precisely establish the digital value associated with the black reference level of the analog video signal input and to eliminate any 60 Hz "hum" that may be present in the environment.
In the prior art circuit shown in FIG. 1, the clamping is performed in two stages. A first fast clamping stage samples the incoming analog video through a burst filter at the time of a clamp signal. The clamp signal occurs during the black interval and subcarrier burst time on the "back porch" of the horizontal sync interval. The burst filter blocks the subcarrier burst so that the black level may be examined free of the influence of the burst.
The sample is taken at the output of the first amplifier in the signal path after either AC or DC coupling. The analog sample and hold circuitry that performs the sampling has an inverting amplifier at its output that returns an inverted version of the sample as feedback to the input of the amplifier. This first analog clamp is relatively fast and typically has a bandwidth and dynamic range great enough to substantially reduce a large 60 Hz component that may be modulating the video due to ground current loops or related phenomena. The output of the first clamp is low-pass filtered to provide anti-aliasing and applied to the input of a second, slower clamping stage that also performs the analog-to-digital conversion.
In this second stage the analog signal is converted to digital and this digital video signal is also sampled at the time of the clamp signal. The result of that sampling is compared to a reference level to produce a feedback signal that is then integrated over a relatively long time constant to produce a low bandwidth clamping action. The feedback signal is a single bit corresponding to an error of one least significant bit (LSB) of black level change, and so only this single bit of correction is provided each sample period. Thus, this stage is limited to correcting very low frequency errors, such as drift due to temperature change.
The first clamping stage shown in FIG. 1 poses certain problems in that both analog burst filters and analog sample and hold circuits are difficult to implement without introducing impedance anomalies and spurious signal components. Analog sample and hold circuits also tend to produce glitches at the times of their turn on and turn off transitions. Moreover, the combination of the first and second stage of clamping requires more circuitry than is desirable if an alternative were available. And, the circuit shown in FIG. 1 is incompatible with automated digital diagnostics.